Third Harmonic Injection with Carrier-Reassignment PWM for a Nine-Level Grid Connected CHB Inverter

Carrier-based pulse width modulation (PWM) techniques are widely employed in cascaded H-bridge (CHB) multilevel inverters due to their computational simplicity and ease of implementation. However, the widely used level-shifted PWM (LSPWM) causes extreme power imbalance across the modules of the CHB inverter, leading to device over-stressing and premature failure. To address this issue, carriers can be re-assigned to different modules over one or multiple cycles in so-called CarrierReassignment PWM (CRPWM) techniques. In particular, a four-quadrant CRPWM technique (Type-A) results in near-perfect power balance across the CHB modules. In three-phase systems, third harmonic injection is a well-known technique to improve DC bus utilization, increase the number of levels in the line-to-line voltage, and enhance power quality by reducing the total harmonic distortion (THD). This paper presents the adaptation of the CRPWM techniques, combined with third harmonic injection, for improved DC-bus utilization and enhanced module power balance in a three-phase grid-connected CHB inverter. Analytical and simulation results show the superior performance of the Type-A CRPWM technique in achieving near-perfect power balance while optimizing DC-bus utilization. The paper also compares the performance of LSPWM, FIFO-CRPWM, and Type-A CRPWM both with and without third-harmonic injection, in a three-phase grid-connected nine-level CHB inverter operating at unity-power factor under closed-loop dq-controller.